John Curl's Blowtorch preamplifier part II

[john curl]

Another photo. RIP both of these guys.

[Esperado]

Originally Posted by john curl

with Noel Lee

The guy who invented the 'Sound of cables ' ?

[Bonsai]

Originally Posted by john curl

Another photo. RIP both of these guys.

When did Jim B pass away?

[SY]

In the past couple weeks.

[Esperado]

Originally Posted by john curl

Another photo. RIP both of these guys.

There is no more evil thermal effects in audio engineer's Paradize.

[scott wurcer]

Originally Posted by kgrlee

JC, if you believed in him, you should have read his datasheet.

The best OPA or even Golden Pinnae stuff, hand carved from solid Unobtainium by virgins, won't stop poor design from losing 10dB of signal noise .. or introducing distortion and instability with poor decoupling and layout.

Mostly explained in the AD797 datasheet.

Yes but there are several MM input pre-amps in production that use equivalent bi-polar parts from us or LT, and then there was one from one of John's white hats (Mitch Cotter) but I think that was MI (Grado). You could use the 1977 National app. note and add A weighting and see what you get.

[john curl]

I am not much in love with MM. I was asked to make a simple and compact phono preamp that originally was going to be put inside the JC-2 as an add-in. I didn't think of using the AD811 at the time, and I still might not use it today, but any added buffer would add cost, space and heat. I made the compromise to not 'stress' the input op amp with a 600 ohm load. Unlike Scott, I only use moving coil cartridges, so I optimized it for that use.

[scott wurcer]

Originally Posted by simon7000

Now You are welcome to do your calculations showing - infinity or -165 or -220 or whatever you want. When you measure a real circuit you will be hard pressed to get -150 (all re 1 V RMS).

ES

What's all the fuss here? I've built characterization jigs and have helped dozens of customers in building low noise instruments where the there is no incursion of noise from the supplies. It's known as engineering.

[john curl]

Scott, the typical Grado cartridge has a DC resistance of 475 ohms. Please add that to your calculations.

[bcarso]

Originally Posted by scott wurcer

Yes but there are several MM input pre-amps in production that use equivalent bi-polar parts from us or LT, and then there was one from one of John's white hats (Mitch Cotter) but I think that was MI (Grado). You could use the 1977 National app. note and add A weighting and see what you get.

I wish we could persuade JA/Stereophile to measure phono noise with the input open, as well as what they do now (input shorted), and then back out the current noise contribution based on various cartridge loads. MM applications in particular, given their high inductances, will be affected significantly.

JC's (and some others') JFET-based preamps have low current noise, as well as very low voltage noise. Most op amp-based designs have high current noise, unless they are JFET input, in which case they usually have rather higher voltage noise.

The datasheet for the TI/nee National LME49710 shows the schematic for a two-amp RIAA preamp, with the implication that it should have good noise performance, but the current noise (1.6pA/sqrtHz midband) will be on the high side for that op amp used at the input. Note that some form of bias current cancellation must be used, as by itself the net bias current is a mere 7nA.